SIST EN 62308:2007

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Zanesljivost opreme - Metode ocenjevanja zanesljivosti (IEC 62308:2006)Zuverlässigkeit von Geräten - Verfahren zur ZuverlässigkeitsbewertungFiabilité de l'équipement - Méthodes d'évaluation de la fiabilitéEquipment reliability - Reliability assessment methods21.020Characteristics and design of machines, apparatus, equipment03.120.01Kakovost na splošnoQuality in generalICS:Ta slovenski standard je istoveten z:EN 62308:2006SIST EN 62308:2007en01-februar-2007SIST EN 62308:2007SLOVENSKI
STANDARD
EUROPEAN STANDARD EN 62308 NORME EUROPÉENNE
EUROPÄISCHE NORM December 2006
CENELEC European Committee for Electrotechnical Standardization Comité Européen de Normalisation Electrotechnique Europäisches Komitee für Elektrotechnische Normung
Central Secretariat: rue de Stassart 35, B - 1050 Brussels
© 2006 CENELEC -
All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 62308:2006 E
ICS 03.120.01; 03.120.99
English version
Equipment reliability -
Reliability assessment methods (IEC 62308:2006)
Fiabilité de l'équipement -
Méthodes d'évaluation de la fiabilité (CEI 62308:2006)
Zuverlässigkeit von Geräten -
Verfahren zur Zuverlässigkeitsbewertung (IEC 62308:2006)
This European Standard was approved by CENELEC on 2006-11-01. CENELEC members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the Central Secretariat or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation under the responsibility of a CENELEC member into its own language and notified to the Central Secretariat has the same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Cyprus, the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and the United Kingdom.
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Foreword The text of document 56/1110/FDIS, future edition 1 of IEC 62308, prepared by IEC TC 56, Dependability, was submitted to the IEC-CENELEC parallel vote and was approved by CENELEC as EN 62308 on 2006-11-01. The following dates were fixed: – latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement
(dop)
2007-08-01 – latest date by which the national standards conflicting
with the EN have to be withdrawn
(dow)
2009-11-01 Annex ZA has been added by CENELEC. __________ Endorsement notice The text of the International Standard IEC 62308:2006 was approved by CENELEC as a European Standard without any modification. In the official version, for Bibliography, the following note has to be added for the standard indicated: IEC 61751 NOTE
Harmonized as EN 61751:1998 (not modified). __________
- 3 - EN 62308:2006 Annex ZA
(normative)
Normative references to international publications with their corresponding European publications
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
NOTE
When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD applies.
Publication Year Title EN/HD Year
IEC 60050-191 1990 International Electrotechnical Vocabulary (IEV)
Chapter 191: Dependability and quality of service - -
IEC 60300-1 -1) Dependability management
Part 1: Dependability management systems EN 60300-1 20032)
IEC 60300-3-1 2003 Dependability management
Part 3-1: Application guide - Analysis techniques for dependability - Guide on methodology EN 60300-3-1 2004
IEC 60300-3-2 -1) Dependability management
Part 3-2: Application guide - Collection of dependability data from the field EN 60300-3-2 20052)
IEC 60300-3-3 -1) Dependability management
Part 3-3: Application guide - Life cycle costingEN 60300-3-3 20042)
IEC 60300-3-4 1996 Dependability management
Part 3: Application guide - Section 4: Guide to the specification of dependability requirements - -
IEC 60300-3-5 2001 Dependability management
Part 3-5: Application guide - Reliability test conditions and statistical test principles - -
IEC 60300-3-9 -1) Dependability management
Part 3: Application guide - Section 9: Risk analysis of technological systems - -
IEC 60300-3-11 -1) Dependability management
Part 3-11: Application guide - Reliability centred maintenance - -
IEC 60300-3-12 -1) Dependability management
Part 3-12: Application guide - Integrated logistic support EN 60300-3-12 20042)
IEC 60812 -1) Analysis techniques for system reliability - Procedure for failure mode and effects analysis (FMEA) EN 60812 20062)
1) Undated reference. 2) Valid edition at date of issue. SIST EN 62308:2007

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Publication Year Title EN/HD Year IEC 61025 -1) Fault tree analysis (FTA) HD 617 S1 19922)
IEC 61078 -1) Analysis techniques for dependability - Reliability block diagram and Boolean methods EN 61078 20062)
IEC 61160 -1) Design review EN 61160 20052)
IEC 61165 -1) Application of Markov techniques EN 61165 20062)
IEC 61508
Series Functional safety of electrical/electronic/programmable electronic safety-related systems EN 61508 Series
IEC 61649 -1) Goodness-of-fit tests, confidence intervals and lower confidence limits for Weibull distributed data - -
IEC 61709 -1) Electronic components - Reliability - Reference conditions for failure rates and stress models for conversion EN 61709 19982)
IEC 61710 -1) Power law model - Goodness-of-fit tests and estimation methods - -
IEC 61713 -1) Software dependability through the software life-cycle processes - Application guide - -
IEC 61882 -1) Hazard and operability studies (HAZOP studies) - Application guide - -
IEC/TR 62380 -1) Reliability data handbook - Universal model for reliability prediction of electronics components, PCBs and equipment - -
NORME INTERNATIONALECEIIEC INTERNATIONAL STANDARD 62308Première éditionFirst edition2006-07 Fiabilité de l’équipement – Méthodes d'évaluation de la fiabilité
Equipment reliability – Reliability assessment methods
Pour prix, voir catalogue en vigueur For price, see current catalogue IEC 2006
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Copyright - all rights reserved Aucune partie de cette publication ne peut être reproduite ni utilisée sous quelque forme que ce soit et par aucun procédé, électronique ou mécanique, y compris la photocopie et les microfilms, sans l'accord écrit de l'éditeur. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the publisher. International Electrotechnical Commission,
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Web: www.iec.ch CODE PRIX PRICE CODE XA Commission Electrotechnique InternationaleInternational Electrotechnical CommissionSIST EN 62308:2007

62308  IEC:2006 – 3 – CONTENTS FOREWORD.7 INTRODUCTION.11
1 Scope.13 2 Normative references.13 3 Terms and definitions.15 4 Abbreviations.17 5 Symbols.17 6 Introduction to reliability assessment.19 6.1 Introductory remarks.19 6.2 Description of reliability assessment.19 7 Management of reliability assessment process.27 7.1 Purpose of reliability assessment.27 7.2 Documentation.39 8 Data needs.39 8.1 Input data.39 8.2 Data sources and types.41 8.3 Data collection, storage, and retrieval.43 9 Reliability assessment methods.43 9.1 Introduction.43 9.2 Similarity analysis.47 9.3 Durability analysis.51 9.4 Sensitivity testing and analysis.53 9.5 Handbook predictions.57 9.6 Limitations of reliability assessment results.61 10 Considerations for selecting reliability assessment methods.61 11 Reliability assessment process improvement.65 11.1 General.65 11.2 Validating reliability assessment results.65 11.3 Improving the reliability assessment process.65
Annex A (informative) Similarity analysis examples.69 Annex B (informative) Durability analysis.93
Bibliography.107
Figure 1 – Methods requiring a reliability assessment as input.27 Figure 2 – Stages of product life cycle.35 Figure 3 – Reliability assessment and improvement process.45 Figure A.1 – Example similarity analysis flowchart.85
62308  IEC:2006 – 5 – Table 1 – Example of constant rate reliability measures.23 Table 2 – IEC Standards providing guidance on methods.29 Table A.1 – Example characteristic differences.83 Table A.2 – Example high-level similarity analysis spreadsheet.87 Table A.3 – Example low-level similarity analysis spreadsheet.89 Table A.4 – Example process difference factor tables.91 Table B.1 – Values for exponent B for equations (B.7) and (B.8).101 SIST EN 62308:2007

62308  IEC:2006 – 7 – INTERNATIONAL ELECTROTECHNICAL COMMISSION ____________
EQUIPMENT RELIABILITY − RELIABILITY ASSESSMENT METHODS
FOREWORD 1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with may participate in this preparatory work. International, governmental and non-governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for Standardization (ISO) in accordance with conditions determined by agreement between the two organizations. 2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international consensus of opinion on the relevant subjects since each technical committee has representation from all interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any misinterpretation by any end user. 4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications transparently to the maximum extent possible in their national and regional publications. Any divergence between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter. 5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any equipment declared to be in conformity with an IEC Publication. 6) All users should ensure that they have the latest edition of this publication. 7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and members of its technical committees and IEC National Committees for any personal injury, property damage or other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is indispensable for the correct application of this publication. 9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent rights. IEC shall not be held responsible for identifying any or all such patent rights. International Standard IEC 62308 has been prepared by IEC technical committee 56: Dependability. The text of this standard is based on the following documents: FDIS Report on voting 56/1110/FDIS 56/1122/RVD
Full information on the voting for the approval of this standard can be found in the report on voting indicated in the above table. This publication has been drafted in accordance with the ISO/IEC Directives, Part 2. SIST EN 62308:2007

62308  IEC:2006 – 9 – The committee has decided that the contents of this publication will remain unchanged until the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in the data related to the specific publication. At this date, the publication will be
• reconfirmed; • withdrawn; • replaced by a revised edition, or • amended.
62308  IEC:2006 – 11 – INTRODUCTION This International Standard describes procedures that are intended for use in assessing the reliability of items based on data from: the market of similar items; and field data and test data from suppliers of components and modules. The results of such assessments are intended for use as inputs to early equipment design decisions such as system architecture selection as well as business decisions such as estimating the cost of warranties or maintenance cost guarantees. Furthermore the results can be used as the initial estimate for input to safety analysis, for example FTA analysis. Modern electronic components and items are so reliable that estimating or verifying their reliability by testing is very difficult, therefore data from the field for previous similar items are often the only way to get an initial estimate of the reliability. Component manufacturers have used this method for years under the name of the “similarity principle”. By emphasising the use of data from previously marketed similar products, and requiring similarity to be documented, the method is a modern alternative to the classical but now obsolete handbook prediction.
Reliability assessment results should be viewed as an early estimate of the probability that the product reliability targets and goals can be satisfied using the chosen architecture, modules, components and maintenance policy. As such, they may be used, for example, to authorize advancement to the next step in product development, or to authorize progress payments, or to proceed with delivery and acceptance of products. Reliability assessment results should never be used to support a claim that the reliability targets, goals, or expectations have been satisfied. The only certain measure of reliability requirement having been met is from service/field performance. This standard describes the uses for reliability assessment results as well as providing a list of IEC standards that require such results as input. The approach to reliability assessment in this International Standard – encourages the equipment manufacturer to consider all relevant information regarding equipment reliability which may include the effects of design and manufacturing processes as well as component selection issues. This is in contrast to more traditional methods that focus on component reliability as the most significant contributor to the equipment reliability; – encourages the equipment manufacturer to define and use the processes that are most effective for the manufacturer’s own equipment; – describes a continuous procedure in which a reliability assessment can be updated as more information becomes available during the life cycle of the equipment. This information may be used to improve both the reliability of the equipment and the effectiveness of the assessment process. This International Standard describes the application of three approaches to reliability assessment, namely: similarity analysis, durability analysis, and handbook predictions. This standard does not, however, provide information on assessing the reliability of software systems but can be used for assessing the reliability of hardware systems containing embedded software.
62308  IEC:2006 – 13 – EQUIPMENT RELIABILITY − RELIABILITY ASSESSMENT METHODS
1 Scope This International Standard describes early reliability assessment methods for items based on field data and test data for components and modules. It is applicable to mission, safety and business critical, high integrity and complex items. It contains information on why early reliability estimates are required and how and where the assessment would be used. Finally, it details methods for reliability assessment and the data required to support the assessment. To estimate durability (life time or wear-out), the physics-of-failure method is used.
Three types of assessment are discussed in detail: – the similarity approach; – models for durability analysis; – handbook methods. Clause 6 provides an introduction to reliability assessment and Clause 7 the management of the process. Clause 8 describes the data needs, sources and types for assessments and Clause 9 provides details of the assessment methods. Annexes A and B provide additional information to aid understanding of the similarity analysis and durability analysis.
This standard is applicable to making reliability estimates for specifications, design, design modification and support engineering. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. IEC 60050-191:1990, International Electrotechnical Vocabulary – Chapter 191: Dependability and quality of service IEC 60300-1, Dependability management – Part 1: Dependability management systems IEC 60300-3-1:2003, Dependability management – Part 3-1: Application guide – Analysis techniques for dependability – Guide on methodology IEC 60300-3-2, Dependability management – Part 3-2: Application guide – Collection of dependability data from the field IEC 60300-3-3, Dependability management – Part 3-3: Application guide – Life cycle costing IEC 60300-3-4:1996, Dependability management – Part 3: Application guide – Section 4: Guide to the specification of dependability requirements SIST EN 62308:2007

62308  IEC:2006 – 15 – IEC 60300-3-5:2001, Dependability management – Part 3-5: Application guide – Reliability test conditions and statistical test principles IEC 60300-3-9, Dependability management – Part 3: Application guide – Section 9: Risk analysis of technological systems IEC 60300-3-11, Dependability management – Part 3-11: Application guide – Reliability centred maintenance IEC 60300-3-12, Dependability management – Part 3-12: Application guide – Integrated logistic support IEC 60812, Analysis techniques for system reliability – Procedure for failure mode and effects analysis (FMEA) IEC 61025, Fault tree analysis (FTA) IEC 61078, Analysis techniques for dependability – Reliability block diagram and boolean methods IEC 61160, Design review IEC 61165, Application of Markov techniques
IEC 61508 (all parts), Functional safety of electrical/electronic/programmable electronic safety-related systems
IEC 61649, Goodness-of-fit tests, confidence intervals and lower confidence limits for Weibull distributed data IEC 61709, Electronic components – Reliability – Reference conditions for failure rates and stress models for conversion IECI 61710, Power law model – Goodness-of-fit tests and estimation methods IEC 61713, Software dependability through the software life-cycle processes – Application guide
IEC 61882, Hazard and operability studies (HAZOP studies) – Application guide IEC 62380, Reliability data handbook – Universal model for reliability prediction of electronics components, PCBs and equipment 3 Terms and definitions For the purposes of this document, the terms and definitions given in IEC 60050-191, together with the following, apply. 3.1
durability analysis analysis of the equipment’s responses to the stresses imposed by operational use, maintenance, shipping, storage and other activities throughout its specified life-cycle in order to estimate its predicted reliability and expected life 3.2
life-cycle time interval between a product’s conception and its disposal
62308  IEC:2006 – 17 – 3.3
similarity analysis structured comparison of the elements of the equipment being assessed with those of predecessor equipment for which in-service reliability data are available 4 Abbreviations ASIC Application specific integrated circuit BITE Built in test equipment COTS Commercial off the shelf FEA Finite element analysis FFOP Failure free operating period FITS Failure per thousand million hours FMEA Failure mode and effects analysis FMECA Failure mode, effects and criticality analysis FRACAS Failure reporting, analysis and corrective action system FTA Fault tree analysis HALT Highly accelerated life test IC Integrated circuit LCC Life cycle costs LRU Line replaceable unit MCTF Mean cycles to failure MTBF Mean time between failures MTBUR Mean time between unit repair
MTTF Mean time to failure MTTR Mean time to restoration/recovery/repair MTTSC Mean time to service call MTTSI Mean time to service interruption MTTWC Mean time to warranty claim RBD Reliability block diagram RCM Reliability centred maintenance RET Reliability enhancement test SRU Shop replaceable unit
5 Symbols λ Constant failure rate of the exponential distribution t Time period of interest f(t) Probability density function F(t) Cumulative distribution function R(t) Reliability function T* Accumulated exposure time
62308  IEC:2006 – 19 – 6 Introduction to reliability assessment 6.1 Introductory remarks
The reliability of an item will often have to be assessed for a range of reasons including the following: a) setting targets and specifications; b) comparing options; c) identifying and prioritising problems; d) indicating fitness for purpose; e) optimizing support (e.g. spares); f) to give input to other analysis (e.g. safety analysis); g) to prioritise areas for improvement with the greatest cost-effectiveness improvement potential. This reliability may be quoted in a number of ways, including for example
– accumulated percentage of failures;
– call rate;
– probability of survival; – failure intensity;
– instantaneous failure rate;
– MTTF; – MTBF. The procedure outlined in this standard is aimed at providing reliability analysts, project managers, risk management engineers, designers, safety and reliability engineers, and logistic support engineers with an assessment method for an early estimate of an item’s instantaneous failure rate. The process for estimating life for items with a wear-out failure characteristic is also included. 6.2 Description of reliability assessment 6.2.1 General information Reliability is not an attribute that can be assigned or measured for a single item. It is a stochastic or probabilistic parameter and therefore it cannot be measured exactly and repeatedly. It therefore has to be estimated from information on the amount of usage (e.g. running hours, cycles of operation, etc.) and the number of failures observed. It should be presented in the form of a confidence statement such as "80 % confidence that the true probability of successfully completing the mission lies between X and Y” or “period of time of interest without failure is between 0,963 and 0,995". An explanation of confidence and confidence intervals can be found in IEC 61649. The classical definition of reliability is the probability of providing a specified performance level for a specified duration in a specified environment. Although such a probability is a useful measure for mission-oriented, low-volume products such as spacecraft, it is rarely a suitable measure for most high-volume products for which reliability relates more to product population than the performance of a single system or a mission. Specifying a single characteristic such as mean time to failure (MTTF) is not sufficient for a product that exhibits a time-dependent failure rate (i.e. non-constant failure rate).
62308  IEC:2006 – 21 – 6.2.2 Constant failure rate reliability measures The general expression for reliability, R(t), is given by
()−=∫∞tdtttRλexp)( (1) where ()tλ is the instantaneous failure rate. Another very useful (general) expression is
dttdFdttdRtf)()()(=−= (2) where f(t) is the probability density function of times to failure. In terms of these quantities the instantaneous failure rate is given by
)()()(tRtft=λ (3) Yet another fundamental general expression is that for MTTF. This quantity is given by
∫∞=0)(MTTFdttR (4) Now when )(tλis constant with time, it should simply be written as λ. Under these circumstances, times to failure follow an exponential distribution and the following relationships hold:
)exp()(ttλ−=R (5)
)exp()(ttλλ−=f (6)
λλ=)(t (7)
λ1MTTF= often denoted by the symbol θ (8) This only holds when λ is constant. Another useful but problematic quantity is the total accumulated number of product-hours, sometimes denoted by T*. Under the assumption of constant failure rate there is no difference from a statistical point of view between accumulating 1 000 000 h by one product, or 1 h by 1 000 000 products. In either case a point estimate of the population failure rate if there is one failure would be 10-6 failures per product-hour. The parameter λ being independent of time is referred to as the constant failure rate. A constant failure rate has many useful properties, one of which is that the mean value of the distribution of the product’s time to failure is 1/λ. For non-repaired items (components), this mean value represents the statistically expected average length of time until product failure, commonly called the mean life or MTTF. This means that 63 % of the items can be expected to fail from time 0 until MTTF and 37 % after the MTTF. Another useful property of the SIST EN 62308:2007

62308  IEC:2006 – 23 – constant failure rate is that it can be estimated from a population as the fractional decrease in the number of surviving items per unit time. However, it should be noted that the exponential distribution is the only distribution for which the failure rate is a constant and that the mean life is not 1/λ(t) when the failure rate is not constant. For repaired items, MTTF is sometimes misunderstood to be the life of the product rather than the reciprocal of the constant failure rate. If a product has an MTTF of 1 000 000 h, it does not mean that the product will last that long (longer than the average human lifetime). Rather, it means that, on average, one of the products will fail for every 1 000 000 product-hours of operation, i.e. if there are 1 000 000 products in the field on average, one of them will fail in 1 h on average. In this case, if product failures are truly exponentially distributed, then on average 63 % of the products will have failed after 1 000 000 h of operation. Products with truly exponentially distributed failures over their entire lifetime almost never occur in practice, but a constant failure rate and MTTF may in some cases be a good approximation to product failure behaviour.
Table 1 – Example of constant rate reliability measures Constant rate measure Mean life equivalent Definition Use Constant failure rate using time MTTF (mean time to failure) Total failures divided by total population operating time Standard measure for reliability predictions when time is the relevant parameter Constant failure rate using cycles or distance instead of time Mean cycles/ km MCTF Total failures divided by total population number of product cycles or distance, e.g. kilometres Standard measure for reliability predictions when usage is more relevant than time. These measures are sometimes converted to time-based measures by specifying an operating profile or duty ratio Constant restoration/repair rate
MTTR (mean time to restoration/ repair) Total restorations/repairs divided by total population operating time Useful for sizing a repair depot or manufacturing repair line Constant replacement rate
MTTR (mean time to replacement) Total replacements divided by total population operating time Used as surrogate for constant failure rate when no failure analysis is available; useful for warranty analysis Constant service or customer call rate MTTSC (mean time to service call) Total service/customer calls divided by total population operating time Customer perception of constant failure rate; useful for sizing support needs Constant warranty claim rate MTTWC (mean time to warranty claim) Total warranty claims divided by warranted population operating time Useful for pricing warranties and setting warranty reserves Constant service interruption rate MTTSI (mean time to service interruption) Total service interruptions divided by total population operating time Customer perception of constant failure rate; may be an availability measure
62308  IEC:2006 – 25 – There are several equivalent ways of expressing the constant rate measures in Table 1. For example, a constant failure rate of 1 % per year is equivalent to 1,1 10-6 h-1, 1 100 FITs, 0,01 failures per unit per year, 1,1 failures per million hours, and 10 failures per 1 000 products per year (assuming replacement, 9,95 failures per 1 000 products per year without replacement). 6.2.3 Repaired and non-repaired item concepts
Specifying a single value, such as MTTF, is not sufficient for a product that exhibits a time-dependent failure rate.
This standard considers similarity analysis for the constant failure rate case as well as for non-constant failure rate. IEC 61649, IEC 61710 and IEC 60300-3-5 give details on statistical methods for non-constant failure rate, including Weibull analysis. Situations may also occur when repaired items, which are restored to functionality after failure, are not repaired to a ‘good-as-new’ condition and so exhibit a non-constant failure intensity. IEC 60300-3-5 provides guidance on non-constant failure rate and non-constant failure intensity. MTTF should be used in case of non-repairable items and MTBF should be used in case of repairable items.
Generally it is recommended to state the failure probability, F(t), of the item instead of stating MTBF or MTTR; however, if MTTF is used it should be used for non-repaired items and MTBF used for repaired items. 6.2.4 Methods for estimating reliability The following is a list of methods commonly used for assessing reliability: − similarity analysis; − durability analysis; − handbook methods. The main benefit of a reliability assessment is the identification of the major contributions to system failure, rather than the accuracy of the absolute prediction. The identification of the sources of unreliability supports the prioritisation of actions and allows modifications to be made to the design at an early stage. This is especially important if components, modules or design solutions are reused from previous products. In this case the assessment method estimates the failure rate to be expected if improvement activities are not made. The accuracy of any prediction is determined by the quality of the data and their similarity to the proposed design and its usage and environment. Unless new technology is being considered, a reliability assessment should be based on appropriate in-service data that are available. Data may be obtained from a number of sources. In order of preference, they are as follows: − the same or similar equipment used in the same or similar operational, physical and support environment;
− data derived from physical and engineering analysis across the range of environmental conditions in which it will be used;
− test data or field data from component or module suppliers; − data from industry or generic sources. Generic data sources need to be used with great caution and with lower confidence in the reliability assessment until such time as they can be replaced with better data. SIST EN 62308:2007

62308  IEC:2006 – 27 – There are many generic and industry specific data sources to support reliability assessments. This standard describes a number of alternative reliability assessment methods that can provide failure rate data from an equipment level down to a functional or piece part level. When selecting a particular methodology for a specific application, a review of the accuracy and limitations of the approach to provide a justification for its usage should be documented. This justification should include the uncertainty and confidence factors associated with the results of the assessment method. This standard does not address software issues but only covers methods for hardware items. However, it can be used for hardware items that contain embedded software. Reliability of the embedded software and its interaction with the hardware must be addressed, which may change the original reliability information. 7 Management of reliability assessment process 7.1 Purpose of reliability assessment 7.1.1 General There are numerous reasons for assessing reliability of an item. Figure 1 illustrates some examples of the activities that require a reliability assessment as an input. For example, to calculate the spares provision for an item in the field, knowledge of the item’s failure rate and exposure time would be necessary.
Reliability assessment Reliability programme planning Customer requirement review Safety assessment FMEA RBD FTA RCM Availability modelling Compliance test planning Formal design reveiw Markov analysis LCC Risk analysis Spares optimisation IEC
1213/06
Figure 1 – Methods requiring a reliability assessment as input
62308  IEC:2006 – 29 – Table 2 presents the IEC references for standards on the methods that require reliability assessment as an input. Table 2 – IEC Standards providing guidance on methods Method IEC standard Analysis techniques for dependability – Guide on methodology IEC 60300-3-1 FTA IEC 61025 FMEA IEC 60812 RBD IEC 61078 Requirements IEC 60300-3-4 Design review IEC 61160 Availability modelling IEC 61078 Spares provision IEC 60300-3-12 R&M programme plan IEC 60300-1 Risk analysis IEC 60300-3-9 Reliability Centred Maintenance IEC 60300-3-11 Software dependability IEC 61713 LCC
IEC 60300-3-3
Safety assessment IEC 61882 Markov techniques IEC 61165 Functional safety IEC 61508 Reliability prediction IEC 62380 A reliability assessment may be needed to fulfil the following tasks: a) Reliability goal assessment – Reliability assessments are used to help assess the probability that the system can satisfy its reliability goals (feasibility study). b) Comparisons of designs and products – Most systems have design implementation options. Tradeoffs have to be made among the various options, and reliability assessment is an important input to these tradeoffs. These options may even affect the system architecture, e.g. the amount and level of redundancy. Since tradeoffs often have to be made early in the design process, the reliability assessment may be very preliminary. However, it is still useful since the important information may be the relative reliability and ranking of design choices rather than a precise quantitative value. c) Method to identify and prioritise potential reliability improvement opportunities – Reliability improvement activities should generally focus on the areas with the greatest opportunity for improvement. A reliability assessment quantifies the opportunity by identifying the relative reliability of various units and by predicting the reliability improvement obtained from a reliability improvement activity. d) Logistics support – Reliability assessments are a key to deciding on spare part provisioning policy and estimating the costs of a warranty policy. They can also be used for the first estimate of life cycle costs.
e) Determining the interval for 'failure finding' and 'function testing' types of maintenance tasks. f) Mission reliability estimation – Missions may have multiple phases with different equipment configurations, and system reliability models can be used for a first estimate of the potential reliability for the entire mission. SIST EN 62308:2007

62308  IEC:2006 – 31 – One further important factor when assessing reliability is ‘when’, i.e. at what stage in the product life cycle. To assess item reliability it is crucial to start estimating early in the product life cycle and update such assessments as more information becomes available, e.g. from test. Similarly if the assessed reliability is not acceptable, then improvement activities have to be started as early as possible in the product life cycle to ensure reliability improvements. Thus, reliability assessment and monitoring reliability growth (see IEC 61014) is crucial to the correct use of reliability assessments. Reliability assessment results are typically used for: – business decisions;
– system architecture decisions; – equipment design decisions; – safety analyses;
– reliability programme planning and monitoring. 7.1.2 Business decisions Examples of business decisions that rely heavily upon the results of reliability assessment include warranty decisions, maintenance cost guarantees and profit sharing agreements, planned design updates, spares provisioning, maintenance scheduling, budgeting and staffing. Applicable measures may be expressed in cost of ownership terms such as service delay and cancellation or operator maintenance burden. Since business decisions often involve proprietary, sensitive or confidential cost information, reliability assessment reports for these decisions should be carefully controlled and may be maintained separately from results for other purposes. Furthermore, the degree that this information is shared between business entities (e.g. customer, supplier, user) should be the subject of business or contractual agreements. Prior to the selection of a reliability assessment method a number of criteria have to be considered; these include – the desired uses of the assessment (why); – the appropriate time in the system life cycle to perform the assessment (when); – which business entity can most capably perform the reliability assessment (who); – the item(s) for which the reliability assessment is to be performed (what); and
– the factors that should be considered in selecting the appropriate reliability assessment method (how). 7.1.3 System architecture decisions System architecture is the high-level description, in functional terms, of the structure chosen to satisfy the design specification. This high-level description ensures that system objectives are understood by all interested parties, all relevant factors are considered in the design, all elements of the design are defined and understood at the appropriate level, all elements of the design are evaluated correctly, and alternative solutions are considered. SIST EN 62308:2007

62308  IEC:2006 – 33 – Examples of system architecture decisions that can be supported by assessment results are as follows: – fault tolerant design and built-in test; e.g. test method, coverage, or frequency; – top level hardware and/or software functional partitioning; – functional partition between modules (block diagram); – redundancy needs; and – maintenance support for prognostics. 7.1.4 Equipment design decisions Examples of equipment design decisions that should be based upon reliability assessment include, but are not limited to” – system design, comparing hardware technologies, e.g. digital processor, digital logic array versus analogue;
– comparing circuit architecture alternatives; – comparing utilization, duty cycle, or electrical stress derating alternatives;
– selecting or eliminating certain components; – deciding on the level of component integration (ASIC-discrete); – comparing packaging and assembly technology, e.g. surface mount versus through-hole; – comparing environmental management techniques, e.g. vibration damping and cooling; and – identifying and correcting design deficiencies in a timely manner based on field and test data of similar components, modules and design. As with system architecture decisions, the reliability assessment results should be used to substantiate equipment design decisions. 7.1.5 Safety assessment Safety assessment is the disciplined approach to identifying system hazards and their causes, and to assessing their risks. Safety assessment relates to the reliability assessment of safety-related functions and components. An output of reliability assessment is failure rate, which is often used in various analyses for safety assessment, for example – fault tree analysis (FTA); – Markov analysis; – event t
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